1. Field of the Invention
This invention relates to a zoom lens suitable for still cameras, video cameras, or digital still cameras, and relates to an image pickup apparatus using the lens.
2. Description of Related Art
Recently, with the trend toward making an image pickup apparatus (camera), such as a video camera or a digital still camera that uses a solid-state image pickup element, compact, or toward making the apparatus highly functional, demands have been made for a zoom lens in which the entirety of a lens system is compact, is superior in performance, and is capable of being easily manufactured.
In this type of camera, various optical members, such as a low-pass filter and a color correction filter, are disposed between a rearmost lens part and an image pickup element, and, accordingly, a lens system with a relatively long back focus is required for an optical system used for it. Further, in a color camera that employs an image pickup element used for color images, an optical system used for it is desired to be superior in telecentric characteristics on an image side, in order to avoid color shading.
Conventionally, a two-unit zoom lens of a so-called short zoom type which consists of two lens units, i.e., a first lens unit having negative optical power (the optical power is the reciprocal of a focal length) and a second lens unit having positive optical power and in which the variation of magnification is performed while changing a lens interval between the two units is known as a zoom lens designed to be compact. In this short zoom type optical system, the variation of magnification is performed by moving the second lens unit of positive optical power, and image-point position compensation that depends on the variation of magnification is performed by moving the first lens unit of negative optical power.
Further, various optical systems that use a large number of plastic lenses are proposed in order to easily manufacture a product having a simple structure. For example, Japanese Laid-Open No. H3-15513 proposes an optical system applied to the short zoom type two-unit zoom lens.
Generally, the short zoom type two-unit zoom lens has a long back focus required for an optical system that uses the above-mentioned solid-state image pickup element, and has difficulty in keeping telecentric characteristics superior.
In order to lengthen the back focus and improve the telecentric characteristics, a three-unit zoom lens system in which a third lens unit having positive optical power is disposed on the image plane side of the short zoom type two-unit zoom lens has been proposed, for example, in Japanese Laid-Open No. S63-135913 (corresponding to U.S. Pat. No. 4,838,666), Japanese Laid-Open No. H7-261083, and Japanese Laid-Open No. 2000-111798 (corresponding to U.S. Pat. No. 6,308,011).
In the three-unit zoom lens proposed in these publications, the number of lenses making up each lens unit is relatively large, and the entire length of a lens system is long. In addition, there was a tendency towards difficulties in manufacturing, as a glass-made aspherical lens is used.
Like the short zoom type two-unit zoom lens, a three-unit zoom lens that consists of lens units of negative-positive-positive optical power that use plastic lenses is disclosed, for example, in Japanese Laid-Open No. H5-323190 (corresponding to U.S. Pat. No. 5,357,374) and Japanese Laid-Open No. H9-21950.
In Japanese Laid-Open No. H5-323190, each lens unit is made up of single plastic lens. Although excellent from a manufacturing viewpoint, improvements must be made to its optical performance from the viewpoint of a zoom lens to be used for a high-pixel solid-state image pickup element of the present day.
In Japanese Laid-Open No. H9-21950, the first and second lens units are moved while the third lens unit is being fixed when zooming, and at least two plastic lenses are used for the first and second lens units.
The zoom lens disclosed in these publications has the following disadvantages.
Since the third lens unit is fixed when zooming, variation of magnification needs to be performed only by the second lens unit, making it difficult to perform aberration correction.
The number of plastic lenses is half or less than the number of all lenses, and there is room for improvement from a manufacturing viewpoint.
Since the nearest-to-object lens of the second lens unit in which an axial light ray is most distant from an optical axis is a glass-made lens, it is difficult to correct spherical aberrations and comatic aberrations.
The present invention has been made in consideration of these conventional examples, and it is an object of the present invention to provide a zoom lens that is suitable for a photographic system using a solid-state image pickup element, small in the number of lenses, compact, and has excellent optical performance.
In order to achieve the above-mentioned object, the zoom lens of the present invention includes three lens units of negative, positive, and positive optical powers in order from an object side to an image side, in which zooming is performed by moving each lens unit in the direction of an optical axis. A first lens unit has at least one positive lens and at least one negative lens. A second lens unit has at least one positive lens and at least one negative lens, and a plastic lens of positive optical power is disposed on a closest-to-object side.
A more concrete form of the present invention will become apparent from embodiments described later.